1. Field of the Invention
The present invention relates to a commutator for a starter motor. More particularly, it relates to an improvement of a molded resinous body constituting the commutator.
2. Discussion of Background
FIG. 1 shows a typical example of an armature for a starter motor.
In FIG. 1, a reference numeral 1 designates an armature firmly connected to an armature shaft 2, a numeral 3 designates armature coils 1 and a numeral 4 designates a commutator. The commutator 4 comprises a plurality of segments 4a electrically connected to the armature coils 3, an insert member or bushing 4b fitted to the armature shaft 2 and a molded resinous body 4c for integrating the segments 4a with the insert member 4b.
In a conventional commutator for a starter motor, asbestos was used for the molded resinous body 4c. However, in reconsideration of adverse affect of the asbestos to the human body, a glass-fiber-reinforced phenol resin has been used for the asbestos.
In the conventional starter motor having the construction as mentioned above, it is used under such conditions that a large electric current flows in a short time and it rotates at a high speed unlike a common type of motor. Accordingly, the commutator used for the starter motor is subjected to a severe change of temperature in a short time, i.e. a heat shock, and a strong centrifugal force. In the commutator in which the glass-fiber-reinforced phenol resin is used for the molded resinous body 4c, however, it is difficult to form pores which are effective to expell gas because the molded resinous body has good wettability of glass to the base resin. Further, since the molded resinous body 4c lacks flexibility under stress due to the rigidity of the glass fibers, cracks are caused in the molded body 4c due to the thermal expansion of gas produced in a non-reacted resin in the molded resinous body 4c and an inner stress caused by a heat shock when a sudden change of temperature occurs.